In Patent Application EP-0 543 465, there is already disclosed a family of compounds which can suitably be used to form materials which can be written and erased by means of heat and whose operating range corresponds to the ambient temperature range.
This family is formed from a network of molecules, comprising:
A) a metal, such as iron Fe.sub.II or F.sub.III, or cobalt Co.sub.II, PA0 B) ligands which are bonded to this metal, such as: PA0 D) and at least one water molecule H.sub.2 O which is not covalently bonded to the metal complex, but whose presence in the network can be attributed to an hygroscopic agent selected from the precursors of the metal nucleus, PA0 E) and a doping agent, which is an aminotriazole of the abridged formula R'-Trz, wherein R' is an amine N--C.sub.n H.sub.2n+1 wherein n is an integer, and having the general formula: ##STR3##
(1) a substituted 1-2-4 triazole of the abridged formula R-Trz, wherein R is an alkyl group represented by the formula C.sub.n H.sub.2n+1 wherein n is an integer, and having the general formula: ##STR1## and (2) a 1-2-4 triazolate of the abridged formula Trz.sup.- having the general formula: ##STR2## C) an anion which is selected from the group formed by BF.sub.4.sup.-, ClO.sub.4.sup.-, CO.sub.3.sup.2-, Br.sup.-, Cl.sup.- PA1 all molecules simultaneously undergo a transition within an aggregate, PA1 or none of the molecules undergoes said transition. PA1 present thermally induced spin transitions and hence can be thermally written and erased, PA1 can be optically read, PA1 exhibit stable spin states, PA1 have a critical temperature in the ambient temperature range, PA1 exhibit a hysteresis effect which has a much larger amplitude than that of the compounds of the above-mentioned document EP 0 543 465, so that a better control of both the writing and erasing operations is possible and, in particular, the temperatures at which writing or erasing take place meet the standards imposed by the manufacturers of display devices, said standards being: PA1 T.ltoreq.5.degree. C. for erasing PA1 T&gt;50.degree. C. for writing.
The general formula of the molecules of the family described in the above-mentioned document can be expressed as follows: EQU M.sub.II [(R-Trz).sub.2 (Trz.sup.-)].sub.1-x (R'-Trz).sub.x A, n H.sub.2 O.
In this formula, M is the metal and A is the anion, ##STR4## are ligands of the triazolate and substituted triazole types, the respective proportions of which are governed by the value x of the concentration, and n is the number of water molecules H.sub.2 O which are bonded in a non-covalent manner to the molecule of the metal complex.
In addition, in this formula the concentration value x is small, so that the proportion of the ligand R'-Trz is small and, in fact, constitutes a doping agent which acts on the intrinsical properties of the material.
The above-mentioned, exemplary molecules which are selected from said general family include: EQU Fe.sub.II [(HTrz).sub.2 (Trz.sup.-)].sub.0.9 (NH.sub.2 -Trz).sub.0.1 (BF.sub.4.sup.-)(6H.sub.2 O)
wherein ##STR5## which are used as ligands and NH.sub.2 -Trz is aminotriazole which is used as a dopant.
A method of preparing compounds of this general family consists in combining a salt of the metal (for example Fe(ClO.sub.4.sup.-).sub.2) with the ligands in an acid solution, thereby forming a precipitate. Subsequently, the precipitate is separated from the solution. Said precipitate is obtained in powdered form. By using an excess of the metal salt, a certain quantity of water is captured.
The compounds of the general family described in EP 0 543 465 exhibit spin transitions, between a low-spin state (LS) and a high-spin state (HS), which are induced only by heat, and which involve electronic and structural changes of the molecules. Said changes bring about an abrupt change of the absorption spectrum of the molecules and hence of the color of the compounds, and the electronic energy levels also being different.
In the low-spin state (LS), the compounds are dark purple, whereas in the high-spin state (HS), these compounds are chalk-white. In addition, each of the high-spin (HS) and low-spin (LS) states is perfectly stable if the compounds are subjected to an average temperature, referred to as average critical temperature, which ranges between the maximum and minimum temperatures in the high-spin state or low-spin state.
Studies of these materials have revealed that, apart from spin transition, they exhibit a delay effect which can be attributed to intermolecular cooperativity, resulting in a hysteresis phenomenon which, dependent upon the material used, may range from several degrees centigrade to several tens of degrees centigrade, between -20 and 100.degree. C., i.e. in a temperature range which is appropriate for industrial applications.
When a thermal perturbation is applied, the cooperative effect brings about that either:
The compounds of the general family disclosed in EP 0 543 465 can also be used as memory materials which can be thermally written and erased and optically read and, thus, can also be used to store information, process information or display information.
Further, EP 0 543 465, discloses that the temperature range in which the compounds operate depends to a substantial extent on the properties of the ligand selected as the dopant.
The spin transition of the known compounds presupposes the existence of two potential wells, one of which corresponds to the low-spin state (LS) and the other corresponds to the high-spin state (HS). These potential wells are separated by a potential barrier. The bottom of each of the potential wells corresponds, respectively, to a first and a second energy level, the energy level of the bottom of the high-spin well being higher than the energy level of the bottom of the low-spin well.
The application of a thermal perturbation to one of the compounds of said family, which has first been brought to a stable low-spin state (LS) in which the electrons are trapped in the first potential well, results in the induction of unstable, excited high-spin states, after which the electrons relax via allowed transitions, while following a principal relaxation path, this path being the path of the second potential well, which corresponds to a perfectly stable high-spin state in a specific temperature range. The electrons remain trapped in the second potential well corresponding to the high-spin state (HS) and do not spontaneously return to the first potential well corresponding to the low-spin state (LS) due to the hysteresis phenomenon. To return to the low-spin state (LS), the compound must be cooled to a temperature which lies well within the hysteresis range.
Meanwhile, the compounds of the family known from the above-mentioned document exhibit too small a hysteresis amplitude in the ambient temperature range in which they are to be used as materials for recording or storing information. For example, exemplary compound No.5 of EP 0 543 465 EQU Fe.sub.II [(HTrZ).sub.2 (Trz.sup.-)].sub.0.9 (NH.sub.2 -Trz).sub.0.1 (BF.sub.4.sup.-)(6H.sub.2 O)
has a critical temperature in the range from 20 to 25.degree. C. and a hysteresis amplitude of only 10 to 20.degree..